What Is [object Object] in JavaScript: Object.prototype.toString

A deeper explanation of [object Object]

Photo by Ilya Pavlov on Unsplash

Whilst working as a JavaScript developer, you might have used the toString method or heard of it at least.

And, if you are passionate to contribute your experiences to an open-source GitHub project, you would also have some opportunities to see how they check an argument’s type.

One day, I saw something that was quite weird — [object Object]. Um, I had no clue what it meant when I saw it for the first time.

I thought it was some sort of console mistake when printing data out, so I used to use console.dir, instead of console.log for objects.

Then, I saw this from MDN:

“Every object has a toString() method that is automatically called when the object is to be represented as a text value or when an object is referred to in a manner in which a string is expected.”

Prerequisites

To better understand what this post will talk about, you should know what prototype and prototype chain are in JavaScript.

TL;DR

Object.prototype.toString is a method that prints out the name of the class that you passed into toString.

There are several values you can get from it, based on ECMAScript’s specifications. This could be helpful when you check the type of a class of your object.

Types of JavaScript Objects

toString is a method for representing …what?

To understand what the heck this means, you should know what data types JavaScript has. Basically, JavaScript has seven primitive data types and other types, Object.

• Boolean
• Null
• Undefined
• Number
• BigInt
• String
• Symbol
• Object

Some of them are instances of Object, and some aren’t. What toString prints out with each…